The present invention relates to communication networks. More particularly, the present invention relates to a method and apparatus for the removal of dataless TDM frames when transporting private line traffic over an ATM network.
Within a telecommunication network, xe2x80x9cprivate linexe2x80x9d circuits (TDM lines) may be used to transport information, including voice and/or data traffic. A private line connection between two points may be used, for example, by a business to connect Local Area Networks (LANs) in geographically distant offices. The private line connection is reserved, and therefore the network provider can assure a high Quality of Service (QOS) in terms of bandwidth and delay. Although private line traffic may not need such a high QOS, existing customer agreements and equipment often require them. A T1 circuit is one example of a private line circuit and provides a maximum transmission speed of 1.544 megabits per second (Mb/s).
In order to provide this QOS, private line traffic is traditionally transported using a Synchronous Transfer Mode (STM) network. A network using Time Division Multiplexing (TDM) is one example of an STM network. Using TDM, each channel of private line traffic is assigned a specific time period, or TDM channel, configured to let the channel carry a desired maximum amount of data information. In this way, the STM network provides a high QOS because each TDM channel, by design, can handle the maximum amount of data information. As a result, data information is generally not lost or delayed. If, however, less than the maximum amount of data information is being sent over a TDM channel, a number of the channel""s assigned time periods are not used, and bandwidth is therefore wasted when no data is being transmitted.
It is also known that private line traffic can be transported via an Asynchronous Transfer Mode (ATM) network. An ATM network uses dedicated-connection switching technology that organizes digital data into 53-byte cells and transmits them over a medium using digital signal technology. Individually, a cell is processed asynchronously relative to other related cells and may be queued before being multiplexed with other cells, from other channels, over a single line, or xe2x80x9clink.xe2x80x9d Because ATM networks are more easily implemented by hardware (rather than software), faster processing speeds are possible. In addition, ATM networks allow for more efficient bandwidth use because different services, such as voice and data, can be statistically multiplexed over the same link.
An AAL adaptation layer packages higher layer information, such as the T1 or E1 circuit information, into the contents of the 53-byte ATM cell. A number of these virtual circuits are then combined for transport over an ATM network link, such as over a single ATM network xe2x80x9cpipe.xe2x80x9d
To maintain the high quality traditionally associated with STM networks, the AAL 1 adaptation layer is used together with Constant Bit Rate (CBR) service (together known as xe2x80x9ccircuit emulationxe2x80x9d). As with TDM, the CBR circuit emulation approach provides a constant guaranteed rate of transfer. That is, a CBR connection allocates enough bandwidth to each channel to support a desired maximum rate. In this way, CBR circuit emulation provides a QOS similar to that of a STM network, but does not provide any statistical multiplexing benefits since cells are still used even when no information is being transported. In other words, with circuit emulation the excess bandwidth that is not used by a customer is not available in the ATM network for other services. This may be a significant amount of unused bandwidth, especially during non-business hours.
U.S. Provisional Patent Application No. 60/114,394 entitled xe2x80x9cMethod and Apparatus for Transporting Private Line Traffic Over an ATM Networkxe2x80x9d discloses a system for transporting private line traffic over an ATM network using, for example, AAL2 multiplexing. To achieve a reduction in bandwidth using this technique, however, TDM frames without data must not be converted into ATM cells and relayed over the ATM pipe.
In view of the foregoing, it can be appreciated that a substantial need exists for a method and apparatus to remove frames without data when transporting private line traffic over an ATM network that allows for statistical multiplexing benefits while providing high quality private line traffic and solving the other problems discussed above.
The disadvantages of the art are alleviated to a great extent by a method and apparatus for removing frames without data when transporting private line traffic over an ATM network. A number of TDM frames comprising a TDM private line circuit are received, and it is determined if a TDM frame contains data. The determination may be based on, for example, the detection of one or more frame delimiters, such as by comparing information in the TDM frame with a predetermined frame delimiter pattern. Information from the TDM frame is placed into an ATM cell only when the TDM frame contains data. The AAL2 adaptation layer can be used to indicate when TDM frames have been removed. The removed frames are then re-inserted at the ATM to TDM interface. A first plurality of TDM private line circuits, such as T1 circuits, may be multiplexed into a first real time variable bit rate (rt-VBR) virtual circuit, and a second plurality of TDM private line circuits may be multiplexed into a second rt-VBR virtual circuit, such that the bandwidths of the first and second rt-VBR virtual circuits are not limited, such as by using AAL2 multiplexing with substantially large SCR, PCR and MBS values. The first and second rt-VBR virtual circuits may then be combined for transport over the ATM network link, and an indication that a TDM frame without data has been removed is used to re-insert the dataless TDM frame.